1. Field of the Invention
The present invention relates to a binder resin for a toner and a toner and in particular to a binder resin for a toner and a toner comprising a specific polyester resin. Also, the invention relates to a binder resin for a toner and a toner, used for electrophotography.
2. Background of the Related Art
With the progress of office automation, the demand for electrophotography-based copying machines and printers has increased rapidly and performance requirements for such equipment have become higher. There is a generally used method which comprises forming an electrostatic latent image on the photo-sensitive material, then developing the image using a toner, transferring the toner image onto a fixing sheet such paper or the like, and heat-pressing the transferred toner image using heat roll (a hot roll fixing method). In this heat roll fixing method, in order to achieve enhanced economical efficiency in power consumption and increased photography speed, and to prevent papers from being curled, a toner superior in fixing properties is needed, which can be fixed at a lower temperature.
In the conventional technique, there has been proposed a method to improve the molecular weight and molecular weight distribution of a binder resin represented by a styrene-acrylic resin or the like. Concretely, it has been tested to have a binder resin of a low-molecular weight and lower fixing temperature.
However, the melting point has been lowered owing to the lowered molecular weight whereas the cohesive force of a resin was also deteriorated simultaneously. Therefore, there has been a problem that an offset phenomenon occurs in the fixing roll.
In order to prevent such a problem, it has been attempted to use a binder resin with a wide molecular weight distribution by mixing a high molecular weight resin and a low-molecular weight resin, or, further to crosslink the high molecular weight portion of the binder resin or the like. With this method, however, it is difficult to satisfy the fixing properties owing to the increased viscosity of the resin.
Meanwhile, as copiers and printers have come to employ a higher speed, high performance requirements in electrified portion have increased. That is, a higher durability is required and a long-term stability is needed for a toner.
A toner using a polyester resin owing to higher density for a binder resin is often proposed, replacing the conventional styrene-acrylic resin. (e.g. publications of JP61-284771A, JP62-291668A JP07-101318B and JP08-3663B, and specification of U.S. Pat. No. 4,833,057, etc.) Most of the cases, main components are a bisphenol-A type derivative and terephthalic acid. A toner (JP08-5947B) using a polyester resin having large number of terephthalic acid and ethylene glycol or 1,4-butanediol derived units is proposed. However, the resulting toner does not solve the above problems.
In order to improve an offset phenomenon, there is a method to add paraffin wax, low-molecular weight polyolefin or the like as a mold release agent to a toner. In JP49-65232A, JP50-28840A, JP50-81342A or the like is disclosed a technique to use a binder resin containing a styrene-acrylic resin. However, the development properties and blocking resistance of toner particles have been deteriorated in some cases whereas an offset phenomenon has been improved. It is confirmed that the mold release agent has little effect when applied to a polyester resin and the developing agent is rapidly depleted when its amount is increased.
In addition to these problems, there are the problems in using a polyester resin as a binder resin for a toner as follows.                Difficult to grind owing to a high density        Easy to cause changes in performance of polyester by humidity due to its hygroscopicity        Easy to produce fine powder as time goes by        Insufficient durability        
It is currently difficult to provide a toner satisfying performance in the low-temperature fixing properties and offset resistance, while in addition, maintaining other performance at a high level as well. In order to provide a toner with high resolution of copying images, it needs to give sufficient electrophotographic features to the toner. Many methods have been attempted heretofore in order to achieve high resolution and high image quality of copying images; however, no perfect methods could be achieved heretofore to fully solve the above problems.
Meanwhile, in recent years, an increase in population has increased the amount of energy used and has depleted resources. In connection therewith, resource saving, energy saving, recycling of resources, etc. have strongly been claimed. As to polyethyleneterephthalate (PET) bottles, individual municipalities have started a recycling activity, and PET bottles have come to be utilized as clothes or containers. In addition, development of newly recycled items is desired. Thus, a toner using a polyester resin has become a strong candidate.